The amount of missing health or missing mana that you will regenerate with a Total Biscuit of Everlasting Will is calculated when consumed: the amount of health (and mana) per second that the Biscuit grants is constant and does not reduce over the duration.
Passive: Receive a Total Biscuit of Everlasting Will at 3:00, 6:00, 9:00 and 12:00. If the inventory is full while receiving a biscuit, the biscuit(s) will appear as soon as a place in the inventory is freed.
Consume a Biscuit to restore 15% of your missing health and 15% of your missing mana over 15 seconds, as well as permanently increasing your maximum mana by 40. You will also gain the increased mana if you sell the biscuit for 30.
Manaless Champions restore 20% missing health instead. Can hold up to 4 biscuits at the same time.
Additional biscuits can be obtained with Kleptomancy, which grants increased maximum mana only in conjunction with this rune.